Diagnosing dampness in a Victorian-era cavity wall

In message snipped-for-privacy@googlegroups.com, Dave N snipped-for-privacy@gmail.com writes

Hmm.. I don't think the Council/Highways have done you any favours by putting surface water in contact with your wall.

How wide is their drainage channel? Could you create a gap filled with pea gravel. Rain splash is at least 6".

What kind of channel? Could it be blocked and overflowing (nit necessarily just outside of your property)?

The channel is just a series of slabs, about 8" wide, with longitudinal grooves in them. A fair amount of water runs past the house in heavy rain but I never experienced any getting into the house or going through the holes in the air bricks. The channel leads to a gully a couple of doors down and then crosses obliquely under the pavement to reach the road. I haven't seen the house in heavy rain recently but the channel was always free flowing when I lived there and my tenant never reported flooding outside (I think he would have told me). I'll try to dig out a picture - if I can't find one I'll take some when I visit tomorrow.

I'll be watching this thread with some interest as I have a slight damp problem in an 18C cottage with rubble filled stone walls. It had a full damp treatment (electric DPC and replastering) when we bought it 30 years ago. This sorted some of the walls (one was only cured by going back to natural stone on the inside with lime pointing, allowing it to breath) but in this one the plaster low down has only just started flaking.

Here's the best picture I could find - it dates from 2004 but the house hasn't changed much since then, just new pointing on the front wall. You can just about see the channel as a line of grooved paving slabs. I was always a bit concerned about rainwater getting into the holes in the air bricks below the front door but never saw it happen.

Picture is at

In message snipped-for-privacy@googlegroups.com, Dave N snipped-for-privacy@gmail.com writes

Are you sure there isn't a slate DPC hidden and bridged by that cement render?

Can't be 100% sure and was wondering, but the house is definitely pre 1875, which I read was when DPCs were first used. What seems odd to me, though, is that there wasn't any damp at all before about 2014.

The damp is only on the inner skin - the outer wall has always remained dry except when hit by driving rain.

Which I think leaves you with the possibility that the cavity is still bridged lower down despite the raking, combined with condensation?

Could be a condensation problem esp.if it's a new tenant. (Different lifestyle) Best tackled at source. Bath/shower/cooking/unventilated tumble dryer/portable gas heater

However even if you rake out the cavity, there has to be somewhere for any water in there to drain away to.

Google tells me 1875 is when DPCs became mandatory. Our farmhouse (certainly pre-1860) has a slate course in 9" brickwork. More likely when Welsh slate became affordable.

Interesting that you have cavity brick walls in a house that old.

Could it be something as simple as a leaking mains water pipe as it enters the front of the property, especially if the council have dug around. or moved, the pipe when installing the drainage channel? Over time a slow drip can cause the brick/stone to become saturated with water. Have you removed any floorboards in the vicinity to see what lies beneath?

The render has become slightly detached in places - maybe the drainage works disturbed it - and water is getting between it and the brickworks. Knock it off and replace it.

Something similar happened here.

Cheers

I also wonder what is actually underneath the new paving and gulley and whether that actually does touch the wall below ground. It sounds like something you need a specialist for, particularly if you need the council to do something to stop it. Brian

I've seen those installed near me, in a vain attempt to redirect water run-off from a main road. Water still seeps down into the ground where the slabs butt up against each other and during heavy rain the slots have nothing like the capacity to channel all the water.

The council has caused your problems. The pavement should not have been done like this.

You need to engage a decent surveyor and then a solicitor IMHO.

That front wall does not look like a cavity wall. There are many bricks laid at 90 degrees to the run of the wall.

is the cavity section behind that render ??

I did think that - it's more like an English bond with all those headers. Or a very inefficient cavity.

I visited the house yesterday and took some pictures - these can be downloaded from

A particular thing to bear in mind is that the wall was dry for 20 years after the council raised the pavement, so the 'wrong' slope in itself didn't obviously cause a damp problem, although it might of course be contributing to it now.

Here are my findings from yesterday:

1. My damp meter gives high readings inside the house up to about 0.8 metres from ground level on the front wall, so just above the level of the bottom of the window, and the readings drop off rapidly above that height. The readings aren't definitive, of course.
2. The wall generally feels slightly damp in the area where the damp meter readings are high, and the damp gets worse towards the front left corner as seen from the front of the house. This is the end where the telephone pole and the white downpipe are.
3. The damp is worst by far inside a low built-in cupboard that houses the electricity meter, and in a small area about 40cm from floor level and 40cm from the party wall was bad enough to get water on my fingers when touching it. This area also had visible mould near it.
4. I dismantled the cupboard and hacked off the residual plaster and some render around the wettest spot. The bricks underneath were relatively dry but still rather damp to touch. I took photos before and after.
5. One of the bricks in the cupboard was loose - I couldn't get it out as the meter board was in the way but this means that the inner skin has effectively had some continuous ventilation into the cupboard. I'm not sure whether this would be a good or bad thing for the damp. I could push this brick inwards, confirming that there is a cavity (I didn't push it far in case I lost it).
6. Very close to ground level near the dampest part of the wall there is a Virgin Media cable going into the house, which I think is new (I'll have to find historic photos from when I lived there to confirm this - I don't remember it). See picture. You will see that there's a bit of mortar alongside it that could disrupt the flow of water along the channel in heavy rain. The flow would be augmented by the downpipe from two roofs discharging immediately above the house about 50cm from the cable.

So here's my best guess: the root cause of the damp might be the new Virgin Media cable, combined with the slope of the pavement. The cable bridges the cavity right at ground level and isn't properly sealed outside, meaning that in heavy rain there could be enough water depth in the drainage channel for water also to trickle into the cable hole and settle in the cavity. When I visited I noticed that the tenant had also left a rubbish sack in a position that would have impeded flow in the channel (the street doesn't have front gardens so no wheelie bins). That might not have been a regular habit, of course, but could have made the water deeper at just the wrong place if he happened to a leave rubbish sacks downstream of the hole and it then rained heavily.

What does the group think of my theory? Could the damp problem just be solved with some sealant around the Virgin Media cable hole and a word with future tenants about rubbish sack placement? And when I come to rebuild the cupboard would it be best to put an air brick inside the cupboard to assist ventilation or could it be better to seal the wall in the cupboard?

Cable crossing cavities should be sloped to direct water outwards (ideally in a conduit). Also when coming from above,they should be bent into a little loop outside so that any water running down the cable drips off outside.

Well that down pipe is definately a crime suspect. The one next to a pole. Normally downpipes like this feed into cast iron pipes with a long slot in the top, inlaid into the pavement and discharging into the road gutter. Sort of a horizontal extension to the downpipe but embedded in the pavement at right angle to your front wall. Another downpipe has a grill, but where does the water go then ?.

Are there are photos of how it looked before the council did the work ?.

If they have removed those inset drainage gullies AND altered the pavement to slope back towards the house the house then where is the water from your house roof supposed to go ?.

I would put this query at the top of your investigative list.

I have tried to find a picture with Google, but without knowing what the correct technical name is, nothing comes up. I have seen them many times in rows of terraced houses opening right onto the street.